Recovery of high viscosity oils by conduction heating



Jan. 30, 1968 D. R. PARRISH 3,366,176

RECOVERY OF HIGH VISCOSITY OILS BY CONDUCTION HEATING Filed April 28,1966 PRODUCTlON OUTLET v I 9-. nl m AIR 4 E i FLUID DAVID R. PARRISHINVENTOR.

ATTORNEY United States Patent 3,366,176 RECOVERY OF HIGH VISCOSITY OILSBY CONDUCTION HEATING David R. Parrish, Tulsa, Okla., assignor toPan-American Petroleum Corporation, Tulsa, Okla, a corporation ofDelaware Filed Apr. 28, 1966, Ser. No. 545,981 9 Claims. (Cl. 166-11)ABSTRACT OF THE DISCLOSURE Where steam flows through well-to-wellfractures in an effort to distribute heat through the formation, theinjected steam bypasses much of the oil. To remedy this, thewell-to-well fracture is first created, and then the fracture is proppedwith a material that will not melt at steam injection temperature, butwill melt at temperatures between steam injection and combustiontemperatures; alternatively the material may be a readily oxidizableone. Then steam is injected until breakthrough occurs and/or oilproducing rates decline. The proppant is then melted or oxidized, andsteam injection or other thermal recovery method continued.

The present invention relates to the recovery of petroleum from anunderground reservoir thereof by means of thermal methods. Moreparticularly it is concerned with a novel process for the recovery ofheavy viscous oils having an API gravity of not more than about 15 frompreferably hard rock reservoirs wherein a conductive heating process ofsuch oils is first effected to reduce the viscosity thereof and thenfollowed by an appropriate fluid injection method.

Specifically, one or more fractures are created, for example, in a hardrock oil-bearing zone to connect injection and producing wells. Thefracture is made in accordance-with known techniques and with a liquidcapable of carrying a propping agent stable at temperatures of the orderof 550 to 650 F. but which melts, burns, or otherwise disintegrates inthe presence or absence of oxygen at temperatures of from about 650 toabout l200 or 1300 F. The oil is then heated by injecting steam into thepropped fracture until steam breakthrough at the producing well(s) oruntil a substantial reduction in oil producing rate is observed.Thereafter air is injected into the hot fracture(s) to increase thetemperature therein causing the props to melt, burn, and/or oxidize,thereby permitting the fracture (s) to at least partially close or heal.Once this is done, steam injection or other fluid injection methods forproducing the oil of reduced viscosity such as, for example,waterflooding or combustion, may be employed.

One of the principal disadvantages of any of the known direct drivethermal recovery processes is that they do not heat the oil ahead of thesteam bank, or burning front, or

in the area around the producing wells. The latter region is extremelyimportant because it controls the oil producing rate. In these processesthe oil must flow through the inter-well area while it is still cold andviscous. For example, to be operable, a direct thermal drive processsuch as in steam flooding, the flow capacity (expressed as kh/ where kis permeability in millidarcys, h is thickness in feet, and #0represents oil viscosity in centipoises) of the reservoir must be atleast about 20. In those reservoirs where all other requirements are metbut kh/n is lessthan about 20, ordinary steam flooding cannot be used.The importance of distributing heat throughout the reservoir andincreasing the value of kh/ in a heavy oil reservoir arises from thefact that kh/ controls the oil producing rate and the pressure drop inthe inter-well area.

While conventional frontal drive steam flooding reduces the viscosity ofthe crude oil in the area immediately ahead of the high temperaturefront and thus provides good displacement efficiencies, the oil in theinter-well area and the critical region around the producing well isunaffected. As a result, the injection pressures can be excessive andproduction rates low.

It has been suggested that the oil can be produced by forming awell-to-well fracture, the injected steam then channels through thefracture, warming all of the oil by heat conduction. Following injectionof a suflicient amount of steam to heat the pay zone, steam injection isshifted from the fracture to the pay so that oil of reduced viscositycan be displaced. There is evidence, however, that this proceduredefeats the purpose of the fluid drive because the injected steam tendsto flow through the fractured portion of the reservoir thus bypassingmuch of the oil, with oil production decreasing sharply at steambreakthrough.

In accordance with the process of my invention, these difliculties areovercome by forming the above-mentioned fractures with a liquid carryinga suitable propping agent ranging, for example, in size from about 10 toabout 40 mesh and preferably from about 10 to about 20 mesh. Thispropping agent may be selected from a relatively wide group of materialswhich are stable and do not melt at temperatures up to about 550 toabout 650 F., but which lose their compressive strength at temperaturesof the order of from about 650 to about 1300 F. As examples of suchmaterials there may be mentioned ground hard fruit stones, nutshells,synthetic or natural resins, etc. Other materials such as glass beads,aluminum, magnesium, and alloys of these and other metals, melting orlosing their compressive strength within the above-mentioned temperaturerange, can also be used.

It is apparent that the operability of my process depends upon theactual existence of one or more fractures extending from the injectionwell to the producing wells. For this reason, the pattern employed for agiven project generally should not cover an area of more than about 5 to10 acres. With areas larger than this, the chances of being able to formconnecting fractures are less than about percent. Most heavy oildeposits for which the process of my invention is applicable are foundat relatively shallow depths, i.e., less than 3,000 feet. At such depthsit is usually much easier to form a horizontal fracture than at greaterdepths. The presence of heavy crude Within the formation limits leak-offof fracturing liquid and thus makes it easier to extend the fracture. Itis not necessary that the fracture be perfectly horizontal, but it isdesirable that the fracture establish communication between theperforated zones of the injection and producing wells.

After one or more of such fractures have been formed connecting thesewells, steam injection can be initiated. At first steam injection shouldbe carried out at the full fracture or boiler capacity to realize themaximum rate of reservoir heating. Eventually, depending upon theinjection rate, steam temperature and distance between wells, steambreakthrough occurs at the producing wells. Steam jection rate, steamtemperature and distance between wells. however, in general, pressuresgreater than about 2000 p.s.i.a. should be avoided since the steamtemperature is in excess of 635 F. and with many of the otherwisesuitable propping agents, their compressive strength is materiallyreduced at temperatures above 635 F. Ordinary steam injection pressuresof 800 to 1000 to 2000 p.s.i.a. are suitable. In any event, in order toachieve satisfactory conductive heating, heat is preferably supplied tothe reservoir at the rate of about 10 to 15 million B.t.u. for each24-hour period.

There are many benefits, of course, from conduction heating of a heavyoil reservoir prior to subjecting the latter to any of several fluidinjection processes. For example, the crude oil from the NuggetFormation in the Winkleman Dome Field, Wyoming, has a viscosity of about900 cps. at the formation temperature of 85 F. If this formation isheated to 200 F. the viscosity of such oil is decreased to 33 cps. Thisreduction in viscosity brings about a 27-fold increase in [Ch/1LProductivity is considerably higher than that which is obtained at thenormal formation temperature of 85 F. The rate of advance of the hightemperature zone in a subsequent steam drive is also increased becauseof the decrease in the temperature difference between injected steam andthe formation and because of the higher injectivities possible in thehigher temperature formation. The rate of heat conduction from thefractures into the surrounding formation decreases with time. On theother hand, the percentage of steam entering the pay zone (as opposed tothe fractures) at the injection well increases because of the reductionin viscosity of the oil in the pay zone. It is undesirable to waste heatby producing steam at the production wells. If the rate of steamcondensation within the fracture and within the pay is not suflicient toprevent saturated steam from being produced, the steam injection rateshould be decreased.

After breakthrough of steam into the producing well or the producingrate declines, air is injected into the formed fractures at a rate offrom about 5,000 to about 50,000 cu. ft./hour. At the temperatureprevailing in the fracture, e.g., 500 to 600 F., ignition isunnecessary, the injected air effecting a rapid oxidation of the oilgenerating more heat, ultimately causing the fracture props to melt,burn, or otherwise disintegrate. Essentially the same result can beobtained by the use of a lean fuelgas-air mixture ignited in thefracture to destroy the propping agent. Alternatively, a mixture of hotgases, e.g., 500 to 600 F. steam and air, may be employed to bring aboutthe necessary temperature in the fracture to overcome the compressivestrength of the props. At the time the fracture closing or healingoperation is begun, the fluid injection pressure is relatively low,e.g., 500 to 800 p.s.i. As the props in the vicinity of the injectionwell and out into the fracture melt, or are destroyed, the fluidinjection pressure will be observed to increase. When such pressure hasincreased, for example, to roughly twice the original pressure, e.g.,1000 to 1500 p.s.i., the fracture or fractures can be considered to havehealed or closed. In other words, healing of the fracture is indicatedby a substantial increase in injection pressure. This fracture healingstep generally takes place primarily within several feet from theinjection well which fact is actually beneficial because as a result ofthis, the subsequently injected steam or other fluid is forced out intothe rock matrix and after traveling through the pay zone for somedistance, finds its way back into the unclosed fracture which stillcommunicates with the producing well thus furnishing a relatively largedrainage area into which the oil of decreased viscosity can flow. Inplace of steam injection after the fracture healing step, one may, ifdesired, employ other fluid injection methods such as, for example, hotor cold water flooding or injection of air to form a combustion frontand producing the oil of reduced viscosity by ordinary in-placecombustion.

In addition to the conditions already mentioned above, it should bepointed out that the pay zones to which the present process isapplicable are preferably at least feet in thickness in order to reducethe amount of heat lost to the cap and base rocks. Preferably, thisshould be a continuous sand since heat used to raise the temperature ofnonpay sections is heat wasted. The process of the present invention isdesigned for those reservoirs where kIz/ is low (less than 20 md.ft./cp.) solely because of high oil viscosity. Formation and crudeproperties should be such that this ratio can be made greater than 100by heating the reservoir to 200 F. As previously indicated,

the present invention is directed to reservoirs containing tars or lowAPI gravity crudes. These materials undergo the greatest decrease inviscosity when heated. To be beneficial the viscosity of the tar or oilsshould preferably undergo at least a 20-fold decrease when heated fromreservoir temperature to about 200 F. This magnitude of viscosityreduction can be achieved with most crudes of gravity less than 15 API.Accordingly, the expression high viscosity oil as used in the presentdescription and claims is intended to mean an oil or tar having agravity of less than about 15 API. A high initial oil content isnecessary to insure an economical process. The reservoir oil contentshould be greater than 780 barrels/acre foot.

The method of my invention will be further illustrated by reference tothe accompanying drawing wherein portions of a heavy oil (12 API)bearing zone 2 is shown in vertical cross section. Oil bearing zone orformation 2 has an initial oil content of about 900 barrels per acrefoot and is at a depth of about 1500 feet. From the earths surface 4, atleast two wells penetrate zone 2 at a suitable spacing, well 6 beingequipped as an injection well and well 8 equipped as a producing well.These wells are shown only diagrammatically; however, it will beunderstood that the equipment not shown in detail may be any of thatconventionally employed and connected in the ordinary manner, both wellshaving one or more strings of casing or tubing suitably cemented orsupported in place, well 6 being connected to a conventional supply ofoxygen or air under pressure flowing through valved line 16 and having avalved line 18 through which steam or other suitable fluid may beintroduced. Well 8 is connected to apparatus for separating gas andliquid and for storing the recovered liquids. Since all of thisequipment may be entirely conventional and no particular types arerequired for this invention, no detailed showing is considerednecessary.

Before steam injection is initiated, one or more horizontal fracturesare created in the formation between be tween wells 6 and 8. Thesefractures may be started in and extended from wells 6 and 8 as shown byfractures 10 and 12. Formation of these fractures can be accomplished asdesired by means and procedures now well known in the hydraulicfracturing art and as no special adaptation of these processes isinvolved in this invention, further detailed description is consideredunnecessary.

With the fractures in zone 2 thus formed, they are held open by means ofnutshells as the propping agent 14, 600 F. steam is injected at apressure of about 1500 p.s.i., at a rate of about 20,000 lbs./hour. Asthis process continues, heat travels into formation 2 above and belowfractures 10 and 12 and by the time steam breaks through into producingwell 8, a major portion of the oil in the inter-well area has beenheated to about 200 F. Next air is injected through valved line 16 intofractures 10 and 12 via well 6 at a rate of about 10,000 cu. ft./hourand at a pressure of about 800 p.s.i. At the temperature conditionsprevailing in the fractures, i.e., about 600 F., additional heat isgenerated thereby initiating combustion of the hot oil. Suchtemperatures are generally in the neighborhood of about 1000" F. andcause the nutshell props to disintegrate wherever the temperature levelapproaches this figure. Such action in turn results in the at leastpartial closing of fractures 10 and 12. The darkened portion shown inthe drawings of these fractures extend for a distance of several feetfrom injectionwell 6 and represent burned or disintegrated props thathave lost their compressive strength, causing that segment of thefracture to close. The occurrence of this condition in fractures 10 and12 is observed by a relatively rapid pressure build up to a valuefrequently as much as twice the original injection pressure. It shouldalso be pointed out that it is contemplated as lying within the scope ofmy invention to burn, melt, or otherwise destroy the entire supply ofpropping agent in the fractures, causing the latter to elfect asubstantially Complete healing thereof throughout the inter-well area.

Once fracture healing has occurred to the extent desired, steam or otherappropriate fluid is then injected into formation 2 through valved line18 and input well 6. The closed or healed fractures and 12 thus forcethe injected fluid to take the path indicated by the arrows, therebypropelling oil of decreased viscosity into producing well 8 via thepartially open fractures as well as through formation 2 and eventuallyout of the system through flow line 20.

It will be apparent from the foregoing description that I have provideda novel method for recovering tars and high viscosity oils heretoforeconsidered essentially unrecoverable by conventional fluid injectionmethods. Thus by conduction heating through propped inter-wellfractures, followed by closing said fractures, and thereafter injectinga suitable fiuid to drive the hot oil to the producing well, I am ableto effect oil recovery over a shorter flood life at lower injectionpressures, higher producing rates, and with decreased heat losses. Also,while I have stressed in the present description that the fracturesshould be formed so that they extend continuously from one well to theother, the advantages of my invention may likewise be realized fromfractures that are not necessarily continuous but which overlap and area relatively short distance away from one another. Accordingly, theexpression inter-well fracture as used in the description and claims isintended to refer either to continuous WelLto-Well or to overlappingfractures.

I claim:

1. In a method for recovering high viscosity oil from an undergrounddeposit thereof penetrated by an injection well and a producing well,

the improvement which comprises first forming at least one inter-wellfracture in said deposit bringing said wells into communication with oneanother, depositing a propping agent in said fracture, said agent havingadequate compressive strength at temperatures up to about 650 F. to holdsaid fracture open, thereafter injecting steam into said fracture at atemperature below about 650 F. and below the temperature at which thespecific propping agent employed loses its compressive strength wherebyheat is transferred by conduction to the oil above and below saidfracture and between said walls thereby substantially reducing theviscosity of said oil,

next increasing the temperature in said fracture to a level sufiicientlyhigh to substantially destroy the compressive strength of at least aportion of said propping agent causing at least a segment of saidfracture to close,

thereafter subjecting the resulting heated deposit to a fluid injectionsecondary recovery method, and recovering oil of reduced viscosity fromsaid producing well.

2. The method of claim 1 wherein the temperature in said fracture isincreased by injecting an oXygen-contain ing gas therein.

3. The method of claim 2 wherein said oxygen-containing gas is a mixtureof steam and air.

4. The method of claim 1 wherein the temperature in said fracture isincreased by burning a lean fuel gas-air mixture therein.

5. The method of claim 1 in which the propping agent employed is in theform of granular nutshells.

6. The method of claim 1 wherein steam is injected into said fractureuntil steam breakthrough into said producing well is obtained.

7. The method of claim 1 wherein said resulting heated deposit issubjected to a steam flooding operation.

8. The method of claim 1 wherein said resulting heated deposit issubjected to a waterfiooding operation.

9. The method of claim 1 wherein said resulting heated deposit issubjected to an underground combustion operation.

References Cited UNITED STATES PATENTS 2,946,382 7/1960 Tek et al.166-11 2,962,095 11/1960 Morse 166-11 3,131,761 5/1964 Scott 166113,180,414 4/1965 Parker 166-40 X 3,221,813 12/1965 Closmann et a1. 16640 X 3,297,088 1/1967 Huitt et al 166--42 X 3,342,263 9/1967 Fischer16642 X STEPHEN J. NOVOSAD, Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,366,176 January 30, 1968 David R. Parrish It is hereby certified thaterror appears in the above numbered patent requiring correction and thatthe said Letters Patent should read as corrected below.

Column 2, line 60, strike out "jection rate, steam temperature anddistance between wells." and insert instead H injection may be effectedover a wide range of conditions, column 6, line 2, for "walls" readwells Signed and sealed this 1st day of April 1969.

(SEAL) Attest:

EDWARD J. BRENNER Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer

